1. Field of the invention
The present invention relates to an apex seal for rotary piston engines and a method for manufacturing the same, more particularly to an apex seal having an improved wear resistance and, strength or toughness, specifically against bending moment.
2. Description of the Prior art
The apex seal for a rotary piston engine is fitted at the apex of the rotary piston and is adapted to make a planetary motion with the piston while being exposed to heated gas and subjected to various forces in the operating chamber. Thus, it is necessary for the apex seal of the rotary piston engines to have a sufficient wear and heat resistances and the like.
In the field of rotary piston engine, conventionally a cast iron of which surface is chilled to form a hardened layer so-called chilled structure by means of an electron beam is employed for the apex seal as disclosed in U.S. Pat. No. 3,658,451 so that an excellent sliding contact property can be obtained between a trochoidal surface of rotor housing and the apex seal.
Recently, there has been increased the requirement for a high power rotary piston engine. For this purpose, it is desirable to provide the apex seal with a low sliding resistance and light weight by providing a compact configuration, for instance by reducing the thickness. It should however be noted that if the thickness of the apex seal is decreased, the load acted on the apex seal will be unduly increased because the apex seal is used under such severe condition as mentioned above.
In this regard, an idea is proposed that an apex seal of which sliding surface is chilled to be hardened may be applied for such high power rotary piston engine. It should however be noted that although the surface portion of the apex seal has an improved wear resistance through the chilling treatment, the boundary portion between the chilled portion and the matrix portion thereof other than the chilled portion forms a coarse martensite structure and becomes brittle by the heat affect induced by the high energy heating beam. The main portion or matrix portion of the apex seal other than the chilled portion is of a bainite structure produced through a mere casting treatment without any additional treatment wherein some alloy elements constituting the apex seal produce a certain segregation to make a heterogeneous structure to thereby decrease the strength. Further the difference between the chilled portion and the matrix portion of the material other than the chilled portion in hardness, and in structure causes the binding strength therebetween to weaken so that the apex seal may be broken during operation of the rotary piston engine.